Coking up of furnace tubes in multi-pass oil furnace tubes is a problem that impacts on the day-to-day operations of a typical oil refinery. For example, in the delayed coking process, a petroleum residuum (also referred to as “feed”) is heated to coking temperature in a tube furnace, and the heated residuum is then passed to a coking drum (often referred to as a “delayed coking drum”) where the heated residuum decomposes into volatile components and delayed coke. The delayed coking process has been used for several decades, primarily as a means of producing useful products from the low value residuum of a petroleum refining operation.
Coker furnaces typically include multiple banks of furnace tubes; banks of furnace tubes are often referred to as “passes”. Two or more passes are typical, e.g., a four pass tube furnace. Each bank of furnace tubes is heated by a burner such as a gas fired heater. For example, a tube furnace with four passes would typically have four independently controlled burners, i.e., one burner per pass. Typically, each burner is controlled by a gas-controller for controlling the amount of gas fed to the burner thereby allowing individual control over the furnace tube temperature of each pass.
The tube furnace heats feed in the form of high boiling petroleum residues to a suitable temperature of about 900° F. The heated feed is directed to a delayed coke drum. During normal operation of the tube furnace the furnace tube of each pass becomes fouled by coke deposits on the interior surface of the tubes. As this fouling process progresses, the furnace efficiency drops, and progressively more severe furnace conditions are required to heat the incoming feed to coking temperature. As a result of this internal furnace tube fouling, it is necessary to periodically decoke the furnace tubes.
A similar problem occurs in multi-pass oil refinery tube furnaces used to heat crude oil prior to entry into downstream fractionator plant, and petroleum oil furnaces used to heat petroleum feed to be fed into downstream vacuum distillation plant; all these tube furnaces, which typically comprise of a plurality of passes (“multi-pass oil refinery furnaces”) can experience coke build up on the inner surfaces of the tubes thereby necessitating some form of decoking process to remove the coke built up inside the furnace tubes of each pass.
There are several methods used to decoke the furnace tubes. In some procedures, the furnace is taken out of service during the decoking procedure. In other procedures, only a part of the tube banks are removed from service. In all cases, production is either halted or reduced during the furnace decoking process.
One decoking procedure, sometimes referred to as online steam spalling involves injecting high velocity steam or water and cycling the furnace tube temperature enough, such as between about 1000° F. and about 1300° F., to cause contraction and expansion of the tube, with resultant flaking off of the accumulated coke deposits. The deposits are then blown from the furnace tubes by steam flow. This procedure can be carried out on a portion of the tube banks while another portion of the tube banks remains in production sending heated feed to the pair of delayed coke drums, one of which receives the heated feed in accordance to their batch-continuous mode of operation.
Another decoking procedure involves injection of air along with the steam at some stage of the decoking; it is also possible to gradually increase the amount of air in the steam until just air is being injected into the furnace tubes, usually one pass at a time because of the risk of overheating and costly damage to furnace tubes. Because the tubes are still very hot during the decoking, the air combusts the coke deposits, such that there is combustion of coke.
In more detail, in a typical oil refinery coking process, feed in the form of high boiling petroleum residues is heated in a furnace to a temperature of typically about 900° F. to provide heated feed which is then fed to one or more coke drums (often called delayed coke drums). A pair of coke drums are alternately filled and emptied, with heated feed being pumped into one of the drums while the other drum is being emptied of coke and prepared for the next filling cycle.
In a typical batch-continuous coking process a coker-module comprises a first coke drum and a second coke drum (respectively labeled as “DRUM 1” and “DRUM 2” in prior art FIG. 1), which operate in parallel such that when the first coke drum is online and being filled with heated feed from a tube furnace (labeled as “DCD Tube Furnace” in prior art FIG. 1), the second coke drum is being decoked to purge and harvest the manufactured coke contained therein. Thereafter, when the first delayed coke drum has reached capacity, the heated feed is switched to the second coke drum that has just previously been purged of its contents, and first coke drum is primed for the decoking process where its contents are purged and harvested. This cyclical process is commonly referred to as batch-continuous or continuous-batch operation.
As noted in U.S. Pat. No. 5,891,310 typical delayed coke drum cycle time is around 18 hours (see prior art FIG. 2) though shorter or longer cycle times are possible depending on the specification of the plant equipment and commercial operation requirements. With the exception of interruptions for events such as maintenance shutdowns, the continuous-batch mode of operation allows a refinery to maintain continuous-batch operation.
Interruptions to the cyclical continuous-batch process described above can occur if the furnace supplying heated feed is shutdown for maintenance. For example, furnaces are shutdown as a result of coke-buildup inside the furnace tubes. Mild coke-buildup occurs in the furnace tubes as oil feed is heated in the tubes. Coke-buildup inside the furnace tubes reduces the operating performance of the furnace.
To achieve normal furnace performance a decoking procedure is carried out. One way of decoking furnace-tubes is steam spalling in which steam is forced through the furnace tubes to remove coke-buildup from the furnace tubes.
Since decoking a furnace can lead to interruptions in the cyclical batch-continuous process there is a need to decoke furnaces in a timely and efficient manner.